1. Field of the Invention
This invention relates to a new combination control valve which by using effective seal systems, a multiple pressure-step trim, suitable main and pilot valving member profiles and a series of labyrinths interposed between the main valving member and its guide, results in excellent control from minimum to maximum throughput, a high rangeability, an effective seal between the upstream and downstream sides of the valve and a perfect seal towards the valve exterior, intrinsic safety of said valve in the case of malfunction of the seal system towards the valve exterior, extreme ease and facility of disassembly and assembly, and a small actuation force requirement.
2. Discussion of the Background
As is well known, devices have to be installed between the primary and secondary superheater in the boiler start-up circuit of a thermoelectric power plant to keep the boiler out of action during plant start-up until the process fluid conditions upstream of such devices reach rated values, to reduce the pressure of the entire start-up flow and finally to allow said process fluid to pass at maximum throughput with low pressure drop.
Initially said devices consisted of two separate valves, one with a control function and the other with an on-off function. Currently, a single valve is used, known as a combination valve, which is able to perform both fuctions. Said combination valve comprises substantially a valve body provided with a fluid inlet and outlet, a first seat on said outlet for a main valving member provided with balancing holes and slidable along a guide by way of interposed seal elements for balancing the relying member, a pilot valving member rigid with the valve stem to cooperate with a second seat present within said main valving member and provided with a spring-loaded shoulder arranged to cooperate with the main valving member, a trim suitable for reducing the fluid pressure, and seal systems between said first seat and said valve body and, respectively, towards the valve exterior.
The known combination valves of the state of the art all present a series of drawbacks, the main one being that they provide neither a secure and effective seal towards the outside nor intrinsic safety in that, in the case of malfunction of the seal system towards the valve exterior, it is impossible to compensate for balancing purposes said outward leakage through said seal elements, thus causing depressurization of the chamber upstream and generating on this latter intolerable upstream forces leading to inevitable breakage.
Again, in addition to assembly and disassembly difficulty, a further drawback of said combination valves is that they are unable to achieve effective control at low loads and hence have a low rangeability, ie a ratio of minimum to maximum control capacity, which is only of the order of 1:250.